1. Field of Invention
The present invention relates to the art of pulp and papermaking from natural, cellulosic materials, More particularly, the invention addresses the science of evaluating the precise degree of delignification to which a cellulosic material has been subjected.
2. Background of the Invention
Natural cellulose for pulp and papermaking is obtained from wood and other herbaceous sources by numerous processes of delignification. Lignin is a constituent of the pitch or gum substance which binds cellulose fiber together to form an integral, composite organism. By means of thermochemical digestion, the gum material is hydrolyzed and separated from the cellulose by water washing and screening.
Two such thermochemical digestion processes widely practiced at present are the kraft and soda processes. By the kraft process, wood is cooked, under heat and pressure, in the presence of a blended solution of sodium hydroxide and sodium sulfide. The soda process is similar but the chemically active compound is exclusively sodium hydroxide.
By either process, the active chemicals react with both the gum and the cellulose,but at different rates. Strong, easily bleached paper fiber is therefore the product of compromise between maximum gum hydrolysis opposed to minimum cellulose destruction.
In the course of commercial pulp production, a working ratio of wood quantity to cooking solution is derived by a combination of analysis and experience. The number of variable parameters relevant to an exact chemical constituency, temperature and time required for a specific digester charge of wood chips to achieve a predetermined degree of gum hydrolysis defies precision. Time or length of cook therefore becomes the final control variable for a specific digester charge.
The consistency to which cellulose is separated from the natural gum directly affects both the strength and whiteness of a paper product laid from the resulting pulp. If cooked too long, the pulp and, hence, paper is weak. If insufficiently cooked, excessive bleaching chemical is required to achieve a predetermined degree of whiteness.
To this end, several standardized tests have been developed to quantify the degree to which gum associated with cellulose has been hydrolyzed. Such tests are predicated on the relative quantity of non-hydrolyzed lignin that remains in natural association with a sample quantity of water-washed pulp. The "Kappa" Number test, defined by TAPPI Standard T-236 OS-76 determines the volumetric quantity of 0.1 Normal potassium permanganate solution consumed by 1 gram of washed pulp in 10 minutes at 25.degree. C. The "Kappa" Number is 50% of the permanganate volume, in milliters, consumed. The percent of lignin remaining in association with the pulp is EQU lignin %=0.147.times."Kappa" Number
Another such residual lignin test is the "K" Number or "Permanganate" Number test which is a specialized permutation or abbreviation of the "Kappa" Number test.
Traditionally, "Kappa" Number and "K" Number tests are performed manually in a chemical laboratory. More recently, automated instruments have been developed to perform the tests automatically: either from pulp "grab" samples from the process flow stream at the washers, for example, or from pulp samples taken "on-line" from a digester blow line. In either case, these automatic "Kappa" Number or "K" Number instruments are computer controlled, electro-mechanical devices that are required to operate in hostile environments. Consequently, such "Kappa" Number or "K" Number instruments are subject to calibration drift.
The prior practice of "Kappa" Number or "K" Number instrument calibration required that a single pulp sample batch be "Kappa" Number or "K" Number tested in both the instrument and the laboratory. The laboratory result was taken as the control test, and the instrument result was adjusted to correspond to it. This calibration procedure was both slow and expensive.
An objective of the present invention, therefore, is to provide a stable sample solution that will yield a consistent "Kappa" Number or "K" Number value regardless of the test procedure used.
Another object of the present invention is to provide a stable calibration solution for automatic "Kappa" Number or "K" Number instruments.